ok, theres a loud humming noise coming from my 99 when it idles. sounds like it's coming from the air box. only does it during idle, when i hit the gas it stops. what is it? MAF?

 

If I remember correctly there is a port on the IAC valve that is covered with a black cap/vent. When the cap falls off it causes the noise. Plug the port with your finger andd see if the noise goes away. A new IAC should fix it but wait until others chime in.

 

this happened to mine and i still had the black cap on it you could try to clean it but you may need a new one there is no good replacement for it besides a ford one i got one from autozone and it made my truck run like crap

 

This happened to my brother, and ever after replacing the IAC, it continued. Turned out to be a bad wire sending a faulty rpm signal to the computer causing the IAC to constantly "adjust", and vibrate, thus making the noise.

 

My '98 4.0 did the same, ended up being the IAC.

 

could be the idler pully thats what happend to me it sounded like my alt was going out but it was the pully cost me like 25$ to fix that could be it idk

 

It's typically the IAC valve. I bet it does it more when it is warm out too, right? Sounds like a fog horn?
Sometimes you can get away with cleaning it, but it both cases it made mine louder (ranger 4L and explorer 4L). Replacing the IAC valve with Napa replacements fixed them.